1. Field of the Invention
The present invention relates to a semiconductor device and a photoelectric conversion apparatus using the same.
2. Prior Art
A bipolar transistor (to be referred to as a BPT hereinafter) is conventionally known as a semiconductor device having a high operation speed, a high gain and a high withstand voltage.
Of conventional BPTs, a DOPOS BPT (Doped Poly Silicon BPT) is known as a BPT having a shallow junction and a high integration degree.
In a conventional BPT of this type, a natural oxide film having a film thickness of about 10 .ANG. is formed between a polysilicon emitter region and a monocrystalline emitter region. This oxide film is broken or formed into balls by annealing at 1,000.degree. C. or more.
FIG. 1 is a schematic sectional view for explaining a detailed arrangement of a conventional BPT. Referring to FIG. 1, a substrate 1 consisting of a semiconductor such as silicon has an n.sup.+ -type buried region 2, an n.sup.- -type region 3 having a low impurity concentration, a p-type region 4 serving as a base region, an n.sup.+ -type region 5 serving as an emitter region, an n-type region 6 serving as a channel stopper, an n.sup.+ -type region 7 for decreasing a collector resistance of a BPT, an oxide film (SiO.sub.2) 8 formed between a polysilicon emitter region and a monocrystalline emitter region, insulating films 101, 102, 103 and 104 for isolating elements, electrodes and wirings, and electrodes 200-1 to 200-3 consisting of, e.g., a metal, silicide and polycide.
The substrate 1 is of n type obtained by doping an impurity such as phosphorus (Ph), antimony (Sb) or arsenic or p type obtained by doping an impurity such as boron (B), aluminum (Al) or gallium (Ga). The buried region 2 need not be formed but is preferably provided in order to decrease the collector resistance. The n.sup.- -type region 3 is formed by an epitaxial technique or the like. An impurity such as boron (B), gallium (Ga) or aluminum (Al) is doped in the base region 4. Polycrystalline silicon is used as the emitter region 5.
In a BPT having the above arrangement, since a flow of holes from the base to the emitter can be blocked by a potential barrier formed in a valence band by a natural oxide film produced in an interface between the base and the emitter, a current gain can be advantageously increased.
FIG. 2 is a potential view schematically showing a potential obtained upon normal operation in a portion in which the natural oxide film is present in a depth direction of an A--A' section of the conventional semiconductor device shown in FIG. 1. Referring to FIG. 2, W.sub.E represents the thickness of an emitter neutral region, and W.sub.B represents a base neutral region. As shown in FIG. 2, a potential barrier is present at a position represented by W.sub.E ' since the natural oxide film is partially formed between the emitter and base regions.
In this conventional semiconductor device, a base current mainly consists of the following components.
A diffusion current of holes from the base to the emitter consists of a current component approximately represented by equation 1-(1) in a portion in which the natural oxide film and therefore the potential barrier are present: EQU J.sub.B1 =(q.multidot.n.sub.i.sup.2 .multidot.D.sub.p /N.sub.E .multidot.L.sub.p) .times.tanh (W.sub.E '/L.sub.p)[exp(V.sub.BE /kT)-1]1-(1)
A base current J.sub.B1 injected from the base to the emitter in a portion in which no natural oxide film is present is represented by the following equation: EQU J.sub.B1 =(q.multidot.n.sub.i.sup.2 .multidot.D.sub.p /N.sub.E .multidot.L.sub.p) .times.coth (W.sub.E /L.sub.p)[exp(V.sub.BE /kT)-1]1-(2)
If a grain size of polycrystals is large enough to satisfy W.sub.B &lt;&lt;L.sub.P, the following equation is obtained: EQU J.sub.B1 '.congruent.(q.multidot.n.sub.i.sup.2 .multidot.D.sub.p /N.sub.E .multidot.W.sub.E) [exp(V.sub.BE /kT)-1] 1-(3)
That is, J.sub.B1 ' is inversely proportional to the emitter thickness W.sub.E and is increased as W.sub.E is decreased. Therefore, as the integration degree of a semiconductor device is increased, J.sub.B1 ' is increased and a current gain h.sub.FE is decreased.
If a grain size of polysilicon is small enough to satisfy W.sub.E .gtoreq.L.sub.P, coth(W.sub.E /L.sub.P).congruent.1 is obtained since a diffusion length L.sub.P is small, and the following equation is obtained: EQU J.sub.B1 '.congruent.(q.multidot.n.sub.i.sup.2 .multidot.D.sub.p /N.sub.E .multidot.L.sub.p) [exp(V.sub.BE /kT)-1] 1-(4)
Since L.sub.P is large, J.sub.B1 ' is increased. L.sub.P changes in accordance with the size or formation conditions of crystal grains of polysilicon, or a base current value is largely influenced by breakdown of the natural oxide film, thereby causing variations in individual BPTs or reducing stability.
A recombination current of electrons injected from the emitter is represented by: ##EQU1##
A collector current is represented by: EQU J.sub.C =(q.multidot.n.sub.i.sup.2 .multidot.D.sub.n /N.sub.B .multidot.L.sub.n)[cosech (W.sub.B /L.sub.N)].times.[exp(V.sub.BE /kT)-1]1-(6)
where q is the electric charge, n.sub.i is the intrinsic semiconductor charge density (Si), N.sub.E is the impurity concentration of the emitter, N.sub.B is the impurity concentration of the base, D.sub.P is the diffusion coefficient of holes, D.sub.N is the diffusion coefficient of electrons, L.sub.P is the diffusion coefficient of holes (.congruent.(D.sub.p .tau..sub.p).sup.1/2), L.sub.N is the diffusion length of electrons (.congruent.(D.sub.N .tau..sub.N).sup.1/2), k is the Boltzmann constant, T is the absolute temperature, and V.sub.BE is the base-emitter forward bias electron. Note that .tau..sub.P and .tau..sub.N represent minority carrier lifetimes of holes and electrons, respectively.
FIG. 3 is a schematic sectional view showing another arrangement of a conventional BPT. A difference between the BPTs shown in FIGS. 1 and 3 is that no natural oxide film is present between a polysilicon emitter region and a monocrystalline emitter region of the BPT shown in FIG. 3. In FIG. 3, the same reference numerals as in FIG. 1 denote the same parts.
In the BPT shown in FIG. 3, a base current becomes a current represented by equation 1-(4). Especially when a junction is shallowed in a semiconductor device having a high integration degree and a high density, the current is increased to decrease h.sub.FE, thereby reducing current drive power. In a BPT in which all emitters consist of monocrystals, h.sub.FE is significantly decreased upon integration at a high integration degree.
In such a conventional BPT, an impurity concentration of an emitter region 5 is 10.sup.19 to 10.sup.21 cm.sup.-3, that of a base region is 10.sup.16 to 10.sup.18 cm.sup.-3, and that of a collector region is 10.sup.- to 10.sup.16 cm.sup.-3.
In this BPT, however, narrowing of a band gap occurs since the impurity concentration of the emitter region is high (10.sup.19 cm.sup.-3 or more). Therefore, an injection efficiency of carriers from the emitter to the base is sometimes decreased (i.e., the current gain h.sub.FE is decreased.
In addition, since the impurity concentration of the base is low, the BPT sometimes cannot normally operate at low temperatures (e.g., 77.degree. K. or less).
In a BPT in which a junction is shallowed in order to increase a packing density, the impurity concentration of the base must be increased to prevent punchthrough between the emitter and the collector. In this case, however, a withstand voltage between the base and the emitter is decreased, and a capacitance between the base and the emitter is increased.
The potential barrier is formed not only in a valence band but also in a conduction band by the oxide film. Therefore, a flow of electrons as majority carriers in the emitter is blocked, and consequently, the current dependency of the current gain h.sub.FE has a gradient.
It is difficult to uniformly produce the natural oxide film. In addition, when the natural oxide film breaks down or is formed into balls by annealing, the base current changes. This phenomenon does not uniformly occur in individual BPTs to cause a variation, thereby varying the characteristics of the BPTs.
Especially in a linear IC, a photoelectric conversion apparatus (area sensor), a line sensor and the like in which a characteristic variation in individual BPTs is considered as a problem, the above phenomenon has a significant influence and therefore is considered as a serious problem.
In a photoelectric conversion apparatus using BPTs as sensor cells, a variation is a main cause of noise and therefore is a serious problem.